This invention relates generally to couplings for gas-containing pipe, and more particularly to a coupling for use in joining two sections of compressed gas insulated transmission line, and a method for utilizing the coupling.
Gas insulated high voltage power transmission lines consist generally of a conductor supported by insulating spacers inside a metal enclosure or pipe which is pressurized with insulating gas, typically sulfur hexafluoride, at pressures such as 45 lbs. per square inch gauge. Presently, these system are constructed from short sections up to 60 feet long which must be joined together. The conductor joint is usually welded or has finger contacts, but the joint of the enclosure or pipe is a major problem as it must be easy and quick to assemble, reliable, gas tight for long periods of time, such as 25 years, low in cost, have some flexibility to account for misalignment of the pipe sections, and provide good electrical contact between adjacent links of pipe.
In particular, it would be an advantage to be able to easily dismantle any section of piping for inspection after a high voltage failure such as an electrical breakdown. These electrical failures are most likely to occur during high voltage field testing following the transmission line installation, but prior to putting the transmission line into service. These electrical breakdowns usually occur because of particle contamination in the system, wherein the particles are either left in the system or produced because of incorrect assembly procedures. If electrical breakdown failures occur, it is necessary to dismantle the failed section to remove the particles and clean any tracking marks on the support insulators caused by the breakdown.
Prior art joints are generally either welded or flanged. Welded joints are relatively low in cost, but have the disadvantage in that they cannot be easily dismantled for inspection purposes. Flanged joints are generally easily dismantable, but are more expensive than welded joints, typically by a factor of three. Also, flanged joints generally require electrical connecting means between pipe sections to provide electrical continuity along the transmission line.
One pipe coupling utilized to sealingly join two sections of gas-containing piping, although it has not been used for gas insulated transmission lines, is the coupling illustrated in U.S. Pat. No. 3,420,554 to Straub. In this coupling, a rubber sleeve is inflated inside a steel case at the joint between the pipes by injecting a special pressurizing fluid through a valve. The rubber sleeve seals tightly against both ends of the pipe. A rubber collar is utilized to keep the rubber sleeve central, and a spring is used to retain the sleeve. These couplings have the advantage of flexibility, the pipe ends need not be specially prepared, and the joints are relatively easy to assemble and dismantle. However, there are disadvantages to this coupling in that it may not seal for the long periods of time necessary for gas insulated transmission lines, electrical connections must be made between the pipe sections, and the pipe sections themselves must be physically retained under pressure. Additionally, the cost of these couplings is of the same order of magnitude as the flanged joints.